Colour forming coupler solutions for use in the photographic industry are solutions of complex organic compounds such as 2-[4'-(4-benzyloxy-phenylsulphonyl) phenoxy]-N-2'-chloro-5'-[4'-(2,5-di-tert amylphenoxy) butyramido] phenyl-2-pivaloyl acetamide in solvents such as 1:1 mixtures of di-N-butyl phthalate and 2(-2 butoxy ethoxy) ethyl acetate. At present these solutions are made by a batch process in which the solution is prepared in an open vessel generally known as a kettle. This process is lengthy, typically taking up to 40 minutes to make the coupler solution, and during this lengthy period there is a marked tendency for degradation to occur in the solution. Moreover, since the solution is formed in an open vessel, it is in contact with the air and there is also a tendency for oxidation to occur in the solution. Additionally the volume of the kettle is generally such that there is significant wastage of material in the event that the process is interrupted or ceased for any reason.
U.S. Pat. No. 4,379,836, published 12 Apr. 1983 describes a process for the production of photographic emulsions consisting of water-immisible or water-insoluble organic substances and an aqueous solution of an organic binder, characterised in that the hydrophilic and hydrophobic phases involved are heated separately, each to its particular temperature above its liquefaction point, and then continuously dosed into a dispersing chamber. The specification of this patent discloses in Example 2 the supply of solid, pulverulent couplers into a solution shaft (single shaft extruder) which is at the same time supplied with tricresyl phosphate heated to 120.degree. C. so that a solution is continuously formed. In this instance heat is supplied by hot solvent rather than by use of e.g. heating jackets.